[SymbolGraph] Omit empty docComment fields

rdar://59500543

Author: bitjammer

lib/SymbolGraphGen/Symbol.cpp

@@ -162,14 +162,18 @@ void Symbol::serializeRange(size_t InitialIndentation,
 }
 
 void Symbol::serializeDocComment(llvm::json::OStream &OS) const {
+  const auto *DocCommentProvidingDecl =
+      dyn_cast_or_null<ValueDecl>(
+          getDocCommentProvidingDecl(VD, /*AllowSerialized=*/true));
+  if (!DocCommentProvidingDecl) {
+    DocCommentProvidingDecl = VD;
+  }
+  auto RC = DocCommentProvidingDecl->getRawComment(/*SerializedOK=*/true);
+  if (RC.isEmpty()) {
+    return;
+  }
+
   OS.attributeObject("docComment", [&](){
-    const auto *DocCommentProvidingDecl =
-        dyn_cast_or_null<ValueDecl>(
-            getDocCommentProvidingDecl(VD, /*AllowSerialized=*/true));
-    if (!DocCommentProvidingDecl) {
-      DocCommentProvidingDecl = VD;
-    }
-    auto RC = DocCommentProvidingDecl->getRawComment(/*SerializedOK=*/true);
     auto LL = Graph->Ctx.getLineList(RC);
     size_t InitialIndentation = LL.getLines().empty()
       ? 0

test/SymbolGraph/Symbols/DocComment/NoDocComment.swift

@@ -0,0 +1,8 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -module-name NoDocComment -emit-module-path %t/NoDocComment.swiftmodule
+// RUN: %target-swift-symbolgraph-extract -module-name NoDocComment -I %t -pretty-print -output-dir %t
+// RUN: %FileCheck %s --input-file %t/NoDocComment.symbols.json
+
+public struct S {}
+
+// CHECK-NOT: docComment